feat(server): add OTA check and firmware download endpoints

Implements /ota/check (version comparison + sig_b64 payload) and
/ota/firmware (binary stream) using the same _impl pattern as
camera_endpoint.py. HMAC auth left commented pending main app wiring.
6/6 tests passing.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

server/ota_endpoint.py

@@ -0,0 +1,102 @@
+# server/ota_endpoint.py
+"""
+OTA firmware update endpoints.
+
+To register in the server main app:
+    from server.ota_endpoint import router as ota_router
+    app.include_router(ota_router)
+
+Route handlers have HMAC auth commented out pending import-path confirmation:
+    from .main import verify_device_hmac  # adjust to actual module
+Then uncomment the Depends lines in ota_check() and ota_firmware().
+
+Firmware artifacts expected under FIRMWARE_DIR (default: server/firmware/):
+    current.bin   — raw firmware binary
+    current.sig   — 64-byte r‖s Ed25519/ECDSA signature
+    manifest.json — {"version": "X.Y.Z", "size": N, "sha256": "hex..."}
+"""
+import base64
+import json
+from pathlib import Path
+
+from fastapi import APIRouter
+from fastapi.responses import FileResponse
+
+FIRMWARE_DIR = Path(__file__).parent / "firmware"
+
+router = APIRouter(prefix="/ota", tags=["ota"])
+
+
+class FirmwareNotFoundError(Exception):
+    pass
+
+
+def _parse_version(v: str) -> tuple:
+    """Parse semver string to comparable tuple; returns (0,0,0) on malformed input."""
+    try:
+        parts = v.strip().split(".")
+        return tuple(int(x) for x in parts)
+    except (ValueError, AttributeError):
+        return (0, 0, 0)
+
+
+def ota_check_impl(current_version: str, firmware_dir: Path = FIRMWARE_DIR) -> dict:
+    """
+    Compare device's current_version against staged manifest.
+    Returns {"update": False} when no update is available or manifest is missing.
+    Returns full update payload when server version is strictly newer.
+    """
+    manifest_path = firmware_dir / "manifest.json"
+    if not manifest_path.exists():
+        return {"update": False}
+
+    manifest = json.loads(manifest_path.read_text())
+    if _parse_version(manifest["version"]) <= _parse_version(current_version):
+        return {"update": False}
+
+    sig_path = firmware_dir / "current.sig"
+    sig_b64 = base64.b64encode(sig_path.read_bytes()).decode()
+
+    return {
+        "update": True,
+        "version": manifest["version"],
+        "size": manifest["size"],
+        "sha256": manifest["sha256"],
+        "sig_b64": sig_b64,
+    }
+
+
+def ota_firmware_impl(firmware_dir: Path = FIRMWARE_DIR) -> bytes:
+    """
+    Return raw firmware binary bytes.
+    Raises FirmwareNotFoundError if current.bin is absent.
+    """
+    bin_path = firmware_dir / "current.bin"
+    if not bin_path.exists():
+        raise FirmwareNotFoundError("No firmware staged")
+    return bin_path.read_bytes()
+
+
+@router.get("/check")
+async def ota_check(
+    version: str,
+    # device_id: str = Depends(verify_device_hmac),  # uncomment when wiring into app
+):
+    """Check whether a firmware update is available for the given device version."""
+    return ota_check_impl(current_version=version)
+
+
+@router.get("/firmware")
+async def ota_firmware(
+    # device_id: str = Depends(verify_device_hmac),  # uncomment when wiring into app
+):
+    """Stream the staged firmware binary to the device."""
+    try:
+        ota_firmware_impl()  # validate existence before streaming
+    except FirmwareNotFoundError:
+        from fastapi import HTTPException
+        raise HTTPException(status_code=404, detail="No firmware available")
+    return FileResponse(
+        FIRMWARE_DIR / "current.bin",
+        media_type="application/octet-stream",
+    )